Peavey XR684 mixer amp

With just the main PA rail AC supply connected, no load,sometimes at switch on will draw sustained >2 amp , so quickly switched off. At 50% mains , sustained 1 amp draw until switched off. Mains Tx ok on its own. DVM-D cold testing of PA devices and bridge rect etc check out reasonable. What to look for cold initially? I may try hot test again with the speaker output pcb disconnected as speaker protector thtristors on there. Initially one output channel failed soon after power up, which could be sp prot failure presumably, mains fuse did not blow though but that >=500W would be going somewhere it shouldn't. Perhaps another 50% mains half second try out and see if a thyristor is hot.

Reply to
N_Cook
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With just the main PA rail AC supply connected, no load,sometimes at switch on will draw sustained >2 amp , so quickly switched off. At 50% mains , sustained 1 amp draw until switched off. Mains Tx ok on its own. DVM-D cold testing of PA devices and bridge rect etc check out reasonable. What to look for cold initially? I may try hot test again with the speaker output pcb disconnected as speaker protector thtristors on there. Initially one output channel failed soon after power up, which could be sp prot failure presumably, mains fuse did not blow though but that

Perhaps another 50% mains half second try out and see if a thyristor is hot.

Peavey typically put a Triac circuit on the speaker outputs to short and blow the amp/fuses instead of the speakers should it go DC.

I tend to use a dual bench current limited supply, and remove the Triac when troubleshooting. Mine only goes to +-30v, but I've never found that a problem, it will still safely find a short etc.

Gareth.

Reply to
Gareth Magennis

What seems odd about this , is it is intermittant. Sometimes it would power up properly , even at 100% mains, ie nothing is permanently defunct. I'll have a closer look at solder joints, if a bias is dropping out.

Reply to
N_Cook

All very odd. One of the crowbars is doing what it should do and heating up a couple of degrees in the process. Randomly about 1 in 5 times at switch on, this crowbar conducts and drops about .9V as a rail voltage is going on that speaker line. Oddly , at 50% mains, whatever is wrong , will drop out after 1 or 2 seconds returning mains draw to normal. One of the output devices failing internally and correcting/"breaker" going-o/c from current passing ? With the heatsinks off and manually switching off the amp, IR thermometer should show which, the next time of powering up. There are bad design/assembly errors in this amp. Firstly what does not at the moment seem to be the problem, TO220 pre-drivers , the legs have shoulders so can normal assembly take advantage of thru-board anchoring of the pins , when soldered. But there is 3mm or so of freeboard under the shoulders and 2 of the 4 such devices are waggling loose , flexing the pcb traces with any movement/vibration of amp. The 4 h/s screws per h/s have no spring washers under the heads, so the polyester of the pcb compresses with age and heat (1998 make), one of the screws was not very tight, I suspect the precursor problem. .

Reply to
N_Cook

** The Kook needs to disconnect the relevant triac crow bar and see if that fixes the problem - could be that it is being triggered by a switch on thump.

Also check the ESR and uFs of the electros in the trigger circuits - if any have lost capacity, triggering becomes more sensitive to large LF signals.

... Phil

Reply to
Phil Allison

For the following designations I'm using PV "XR power amp" schematic as only preA seems available for the XR684

Two base connected o/p 2SA1302 ,only, of the R ch are heating up for a second,driven from Q213(E) . At this stage don't know if an internal problem to one of those pair , or as quite accurate "timing" of excursion, I'll put a monitor on Q213{B} as its probably a further back excursion in the splitting area , involving a cap perhaps to give the timing. Enough for today

Reply to
N_Cook

I swapped the speaker leads over on the outlet pcb and the same 2 output transistors heated up , so verified not a crowbar problem. First guess (1 to 2 sec timing) its a problem with or associated with one of the 22uF 50V caps in the splitter section. At least with no minor supplies connected , its not an preceeding op-amp problem.

Reply to
N_Cook

Looks as though the "monostable " timing is from R254 and C230, set by the output swinging fully negative and reset after the cap discharges. Normal bias Q213(B) is -1V and goes to -3.3V in DC swing state. Don't know what is causing the initial disturbance to the bias, but previously with amps used or stored in damp conditions, I've found it due to leakage in minor Qs. Owner said its stored indoors and never used outdoors but rust spots on metalwork and some pcb CuCarbonate green spots, say otherwise. Looks like some suck it and see zener clamps to localise down.

Reply to
N_Cook

I changed TO92 Q208 2N5400 for MPSA92 and no change in crowbarring. Before wasting any more time on this I thought I'd remove the crowbar caps, just in case both were equally bad, so nullifying swap-over test, but only one PA had enough switch-on instability to trigger either of the crowbars. With hindsight these Nichicon VP(M) caps ,2.2uF 50V are much the same as polarised 2uF 50V caps, suspiciously small. Simple DVM-R test showed apparent charge time of a 100uF cap, so iffy. ESR test showed reasonable ESR, but RLC test was interesting. Initially , for both caps the same, read 1uF then over 10 seconds dropped to .9uF and continued slowly decrreasing exponentially. So something in the chemistry gone wrong?, presumably cooked or something ,with only 50V rating. Replaced with 2.2uF 250V polyester caps on the other side of the board, cable-tied down. I know overkill but 100V ones not laying around and plenty of space on the track side of the output pcb. Tried 30 switch-ons and no crowbarring and repeated on the other output.

Reply to
N_Cook

** I told you to DISCONNECT the crowbars & look carefully at those electros....

.... Phil

Reply to
Phil Allison

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